In order for phenylalanine hydroxylase to catalyze a fully uncoupled reaction, the enzyme must be highly activated and it must have a substrate analogue, such as tyrosine, occupying the catalytic site. Pure phenylalanine hydroxylase catalyzes the unusual reaction of hydroxylation of p-methyl-phenylalanine to p-hydroxymethyl- phenylalanine. The rate-limiting step is not the same as it is for the hydroxylation of phenylalanine. Phenylalanine hydroxylase in hepatoma cells is in a highly activated state.